Water is one of several nuisance contaminants in coal that affect its value since the presence of water leads to increased transport costs and a reduction in the calorific value of the coal resulting in higher fuel consumption per unit of output. In a coal fired power station, for example, the coal is traditionally dried within the combustion space during firing and the heat needed to dry the coal and enable progress towards its ignition temperature is not available for steam generation and is wasted.
The moisture content of coal falls into two broad types: inherent moisture and surface moisture. The inherent or internal moisture of coal is water in micro-pores and micro-capillaries within coal particles that was deposited within the coal during the coal's formation. Surface adsorption moisture of coal is water that forms a layer only on the surfaces of the coal particles. Reduction of both types of moisture is traditionally undertaken using heat delivered in the boiler or, less commonly, in an external dryer. The surface moisture is largely removed by the application of heat, alone or with mechanical pressure, reducing the coal moisture content from as high as, say, 60% down to more moderate levels of the order of 30%. An example of one such brown coal thermal drying and milling process is described in European patent EP 0579214.
The heat required to remove surface moisture by thermal drying is significant and therefore non-thermal methods of drying are welcome. Benefits of non-thermal methods of drying include reductions in fuel consumption and may include reductions in atmospheric emissions of harmful pollutants, including Sulphur Dioxide, Carbon Dioxide, Chlorine, Mercury and others. It is, inter alia, an object of the present invention to s provide a new system suitable for efficient and cost-efficient drying of brown coal or other carbonaceous solid fuel materials to substantially remove surface moisture in a non-thermal manner, i.e. substantially without applying heat energy to the material.